Functional LnFe-NxHy Models of Biological N2 Fixation Nitrogenase (N2ase) is a metalloenzyme that mediates biological nitrogen fixation and is essential to sustain life. As such, the study of nitrogenase attracts intense scrutiny among the biology and chemistry communities. Nonetheless, the mechanism by which nitrogenase enzymes promote the biological reduction of nitrogen under ambient conditions remains a fascinating and unsolved problem. The broad goal of our research is to evaluate the mechanisms by which a single iron site is able to mediate catalytic N2 reduction in synthetic model systems and, by extension, in biology. The proposed program is to design and study biomimetic Fe-NxHy model complexes to address this goal. Our experimental approach stresses functionally, rather than structurally, faithful models of the iron-molybdenum cofactor (FeMoco). Low molecular weight Fe-NxHy complexes will be developed to explore iron sites in local 4- and 5-coordinate geometries that may accommodate dinitrogen and other NxHy functionalites. We posit these geometries as relevant to Fe-NxHy intermediates of the FeMoco. By analogy to the modes of biocatalytic O2 reduction, two limiting mechanisms will be evaluated. The first is an alternating mechanism, where successive protonations occur at the distal and proximal N-atoms of the Fe-N?N subunit in an alternating fashion (e.g., Fe-N=NH ? Fe-NH=NH ? Fe-NH-NH2 ? Fe-NH2-NH2 ? Fe-NH2 + NH3). The second is a distal mechanism, where complete protonation at the distal N-atom precedes protonation at the proximal N-atom (e.g., Fe-N2 + 3 e- + 3 H+? Fe?N + NH3). We will use synthetic model complexes to understand how the nuclearity, local geometry, and electronic structure of Fe- NxHy species control their relative stabilities and reactivity patterns. The intent is to apply this knowledge to design model systems that can facilitate catalytic N2ase activity. Regardless of the precise mechanism for nitrogen reduction at the FeMoco, its ultimate solution will require comparison of spectroscopic data from the cofactor to related data obtained for well-defined model complexes. We will collaborate with researchers that specialize in spectroscopic studies of the FeMoco to make such comparisons. In sum, the functional Fe-NxHy model chemistry proposed will continue to play a critical role alongside current biochemical, spectroscopic, and theoretical model studies aimed at unraveling the chemical mechanism of biological nitrogen fixation.